Load control device for a light-emitting diode light source

ABSTRACT

A load control device for controlling the amount of power delivered to an electrical load is able to operate in a normal mode and a burst mode. The load control device may comprise a control circuit that activates an inverter circuit during active state periods and deactivates the inverter circuit during inactive state periods. The control circuit may operate in the normal mode to regulate an average magnitude of a load current conducted through the electrical load to be above a minimum rated current. The control circuit may operate in the burst mode to adjust the average magnitude of the load current to be below the minimum rated current. The control circuit may adjust the average magnitude of the load current by adjusting the length of the inactive state periods while holding the length of the active state periods constant.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/219,428, filed Dec. 13, 2018, which is a continuation of U.S. patentapplication Ser. No. 15/857,271, filed Dec. 28, 2017, issued as U.S.Pat. No. 10,194,501 on Jan. 29, 2019, which is a continuation of U.S.patent application Ser. No. 15/399,694, filed Jan. 5, 2017, issued asU.S. Pat. No. 9,888,540 on Feb. 6, 2018, which is a continuation of U.S.patent application Ser. No. 15/142,876, filed Apr. 29, 2016, issued asU.S. Pat. No. 9,565,731 on Feb. 7, 2017, which claims the benefit ofProvisional U.S. Patent Application No. 62/155,871, filed May 1, 2015,the disclosures of which are incorporated herein by reference in theirentireties.

BACKGROUND

Light-emitting diode (LED) light sources (e.g., LED light engines) areoften used in place of or as replacements for conventional incandescent,fluorescent, or halogen lamps, and the like. LED light sources maycomprise a plurality of light-emitting diodes mounted on a singlestructure and provided in a suitable housing. LED light sources aretypically more efficient and provide longer operational lives ascompared to incandescent, fluorescent, and halogen lamps. An LED drivercontrol device (e.g., an LED driver) may be coupled between analternating-current (AC) power source and an LED light source forregulating the power supplied to the LED light source. The LED drivermay regulate either the voltage provided to the LED light source to aparticular value, the current supplied to the LED light source to aspecific current value, or may regulate both the current and voltage.

LED light sources are typically rated to be driven via one of twodifferent control techniques: a current load control technique or avoltage load control technique. An LED light source that is rated forthe current load control technique is also characterized by a ratedcurrent (e.g., approximately 350 milliamps) to which the peak magnitudeof the current through the LED light source should be regulated toensure that the LED light source is illuminated to the appropriateintensity and color. In contrast, an LED light source that is rated forthe voltage load control technique is characterized by a rated voltage(e.g., approximately 15 volts) to which the voltage across the LED lightsource should be regulated to ensure proper operation of the LED lightsource. If an LED light source rated for the voltage load controltechnique includes multiple parallel strings of LEDs, a current balanceregulation element may be used to ensure that each of the parallelstrings has the same impedance so that the same current is drawn in eachparallel string.

The light output of an LED light source can be dimmed. Methods ofdimming LEDs include a pulse-width modulation (PWM) technique and aconstant current reduction (CCR) technique, for example. Pulse-widthmodulation dimming can be used for LED light sources that are controlledin either a current load control mode/technique or a voltage loadcontrol mode/technique. In pulse-width modulation dimming, a pulsedsignal with a varying duty cycle is supplied to the LED light source. Ifthe LED light source is being controlled using the current load controltechnique, the peak current supplied to the LED light source is keptconstant during an on time of the duty cycle of the pulsed signal.However, as the duty cycle of the pulsed signal varies, the averagecurrent supplied to the LED light source also varies, thereby varyingthe intensity of the light output of the LED light source. If the LEDlight source is being controlled using the voltage load controltechnique, the voltage supplied to the LED light source is kept constantduring the on time of the duty cycle of the pulsed signal in order toachieve the desired target voltage level, and the duty cycle of the loadvoltage is varied in order to adjust the intensity of the light output.Constant current reduction dimming is typically used when an LED lightsource is being controlled using the current load control technique. Inconstant current reduction dimming, current is continuously provided tothe LED light source. The DC magnitude of the current provided to theLED light source, however, is varied to thus adjust the intensity of thelight output. Examples of LED drivers are described in greater detail incommonly-assigned U.S. Pat. No. 8,492,987, issued Jul. 23, 2010, andU.S. Patent Application Publication No. 2013/0063047, published Mar. 14,2013, both entitled LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODE LIGHTSOURCE, the entire disclosures of which are hereby incorporated byreference.

Dimming an LED light source using traditional techniques may result inchanges in light intensity that are perceptible to the human vision.This problem may be more apparent if the dimming occurs while the LEDlight source is near the low end of its intensity range (e.g., below 5%of a maximum intensity). Accordingly, methods and apparatus for finetuning the intensity of an LED light source may be desirable.

SUMMARY

As described herein, a load control device for controlling a loadcurrent conducted through an electrical load may comprise a loadregulation circuit and a control circuit. The load regulation circuitmay be configured to control the magnitude of a load current conductedthrough the electrical load in order to control the amount of powerdelivered to the electrical load. The load regulation circuit maycomprise a switching device. The switching device may be controlled bythe control circuit to operate in an active state during active stateperiods and in an inactive state during inactive state periods. Thecontrol circuit may be configured to operate in a normal mode and aburst mode, and to control the average magnitude of the load currenttowards a target load current. The normal mode may be applied when thetarget load current is between a maximum rated current and a minimumrated current. The burst mode may be applied when the target loadcurrent is below the minimum rated current. Further, the burst mode maybe characterized by a plurality of burst mode periods each comprisingone of the active state periods and one of the inactive state periods.

During the normal mode, the control circuit may be configured toregulate the average magnitude of the load current by driving theswitching device between different operating states to regulate theaverage magnitude of the load current. The different operating statesmay comprise a conductive state and a non-conductive state, for example.During the burst mode, the control circuit may be configured to adjustthe average magnitude of the load current by driving the switchingdevice between the different operating states during the active stateperiods and stopping driving the switching device between the differentoperating states during the inactive state periods. The control circuitmay be configured to adjust the average magnitude of the load current byadjusting the lengths of the inactive state periods and/or the activestate periods. The control circuit may be configured to adjust thelength of the inactive state periods in one or more of the burst modeperiods while holding the length of the active state periods constant(e.g., until a maximum amount of adjustment has been made to the lengthof inactive state periods). The one or more burst mode periods may beadjacent to each other or may be separated by another burst mode period(or a plurality of burst mode periods). The control circuit may beconfigured to adjust the length of the active state periods and thelength of the inactive state periods in a succeeding burst mode period.The control circuit may repeat the foregoing adjustment steps if furtheradjustment is desired. The amounts of adjustment made to the lengths ofthe inactive state periods and the active state periods may bedetermined such that fine tuning of the load current may be achieved.The determination may be made in real time or based on data stored inmemory.

Also described herein are methods for controlling a load currentconducted through an electrical load. The control may be applied indifferent operating modes including a normal mode and a burst mode.During the normal mode, an average magnitude of the load current may beregulated towards a target current by driving a switching device betweendifferent operating states. For example, the switching device may bedriven between a conductive state and a non-conductive state to regulatethe average magnitude of the load current towards the target current.During the burst mode, the average magnitude of the load current may beadjusted to the target current over a plurality of burst mode periods.Each of the burst mode periods may include an active state period and aninactive state period. The switching device may be driven between thedifferent operating states during the active state period of each of theplurality of burst mode periods. The switching device may not be drivenbetween the different operating states during the inactive state periodof each of the plurality of burst mode periods. The length of theinactive state period may be adjusted in at least a subset of theplurality of burst mode periods while the length of the active stateperiod may be held constant. The length of the active state period mayalso be adjusted, for example, by an active state adjustment amount inat least one of the plurality of burst mode periods. The length of theinactive state period may be adjusted until a total amount of adjustmentis equal to approximately a threshold amount before the length of theactive state period is adjusted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a light-emitting diode (LED)driver for controlling the intensity of an LED light source.

FIG. 2 is an example plot of a target load current of the LED driver ofFIG. 1 as a function of a target intensity.

FIG. 3 is an example plot of a burst duty cycle of the LED driver ofFIG. 1 as a function of the target intensity.

FIG. 4 is an example state diagram illustrating the operation of a loadregulation circuit of the LED driver of FIG. 1 when operating in a burstmode.

FIG. 5 is a simplified schematic diagram of an isolated forwardconverter and a current sense circuit of an LED driver.

FIG. 6 is an example diagram illustrating a magnetic core set of anenergy-storage inductor of a forward converter.

FIG. 7 shows example waveforms illustrating the operation of a forwardconverter and a current sense circuit when the intensity of an LED lightsource is near a high-end intensity.

FIG. 8 shows example waveforms illustrating the operation of a forwardconverter and a current sense circuit when the intensity of an LED lightsource is near a low-end intensity.

FIG. 9 shows example waveforms illustrating the operation of a forwardconverter of an LED driver when operating in a burst mode.

FIG. 10 is a diagram of an example waveform illustrating a load currentwhen a load regulation circuit is operating in a burst mode.

FIG. 11 is an example plot showing how a relative average light levelmay change as a function of the number of inverter cycles included in anactive state period when a load regulation circuit is operating in aburst mode.

FIG. 12 shows example waveforms illustrating a load current when a loadregulation circuit of an LED driver is operating in a burst mode.

FIG. 13 is an example of a plot relationship between a target loadcurrent and the lengths of an active state period and an inactive stateperiod when a load regulation circuit of an LED driver is operating in aburst mode.

FIG. 14 is a simplified flowchart of an example procedure for operatinga forward converter of an LED driver in a normal mode and a burst mode.

DETAILED DESCRIPTION

FIG. 1 is a simplified block diagram of a load control device, e.g., alight-emitting diode (LED) driver 100, for controlling the amount ofpower delivered to an electrical load, such as, an LED light source 102(e.g., an LED light engine), and thus the intensity of the electricalload. The LED light source 102 is shown as a plurality of LEDs connectedin series but may comprise a single LED or a plurality of LEDs connectedin parallel or a suitable combination thereof, depending on theparticular lighting system. The LED light source 102 may comprise one ormore organic light-emitting diodes (OLEDs). The LED driver 100 maycomprise a hot terminal H and a neutral N. The terminals may be adaptedto be coupled to an alternating-current (AC) power source (not shown).

The LED driver 100 may comprise a radio-frequency interference (RFI)filter circuit 110, a rectifier circuit 120, a boost converter 130, aload regulation circuit 140, a control circuit 150, a current sensecircuit 160, a memory 170, a communication circuit 180, and/or a powersupply 190. The RFI filter circuit 110 may minimize the noise providedon the AC mains. The rectifier circuit 120 may generate a rectifiedvoltage V_(RECT).

The boost converter 130 may receive the rectified voltage V_(RECT) andgenerate a boosted direct-current (DC) bus voltage V_(BUS) across a buscapacitor C_(BUS). The boost converter 130 may comprise any suitablepower converter circuit for generating an appropriate bus voltage, suchas, for example, a flyback converter, a single-ended primary-inductorconverter (SEPIC), a Ćuk converter, or other suitable power convertercircuit. The boost converter 130 may operate as a power factorcorrection (PFC) circuit to adjust the power factor of the LED driver100 towards a power factor of one.

The load regulation circuit 140 may receive the bus voltage V_(BUS) andcontrol the amount of power delivered to the LED light source 102, forexample, to control the intensity of the LED light source 102 between ahigh-end (e.g., maximum) intensity L_(HE) (e.g., approximately 100%) anda low-end (e.g., minimum) intensity L_(LE) (e.g., approximately 1-5% ofthe high-end intensity). An example of the load regulation circuit 140may be an isolated, half-bridge forward converter. An example of theload control device (e.g., LED driver 100) comprising a forwardconverter is described in greater detail in commonly-assigned U.S.patent application Ser. No. 13/935,799, filed Jul. 5, 2013, entitledLOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODE LIGHT SOURCE, the entiredisclosure of which is hereby incorporated by reference. The loadregulation circuit 140 may comprise, for example, a buck converter, alinear regulator, or any suitable LED drive circuit for adjusting theintensity of the LED light source 102.

The control circuit 150 may be configured to control the operation ofthe boost converter 130 and/or the load regulation circuit 140. Anexample of the control circuit 150 may be a controller. The controlcircuit 150 may comprise, for example, a digital controller or any othersuitable processing device, such as, for example, a microcontroller, aprogrammable logic device (PLD), a microprocessor, an applicationspecific integrated circuit (ASIC), or a field-programmable gate array(FPGA). The control circuit 150 may generate a bus voltage controlsignal V_(BUS-CNTL), which may be provided to the boost converter 130for adjusting the magnitude of the bus voltage V_(BUS). The controlcircuit 150 may receive a bus voltage feedback control signal V_(BUS-FB)from the boost converter 130, which may indicate the magnitude of thebus voltage V_(BUS).

The control circuit 150 may generate drive control signals V_(DRIVE1),V_(DRIVE2). The drive control signals V_(DRIVE1), V_(DRIVE2) may beprovided to the load regulation circuit 140 for adjusting the magnitudeof a load voltage V_(LOAD) generated across the LED light source 102and/or the magnitude of a load current I_(LOAD) conducted through theLED light source 120. By controlling the load voltage V_(LOAD) and/orthe load current I_(LOAD), the control circuit may control the intensityof the LED light source 120 to a target intensity L_(TRGT). The controlcircuit 150 may adjust an operating frequency f_(OP) and/or a duty cycleDC_(INV) (e.g., an on time T_(ON)) of the drive control signalsV_(DRIVE1), V_(DRIVE2) in order to adjust the magnitude of the loadvoltage V_(LOAD) and/or the load current I_(LOAD).

The current sense circuit 160 may receive a sense voltage V_(SENSE). Thesense voltage V_(SENSE) may be generated by the load regulation circuit140. The sense voltage V_(SENSE) may indicate the magnitude of the loadcurrent I_(LOAD). The current sense circuit 160 may receive asignal-chopper control signal V_(CHOP) from the control circuit 150. Thecurrent sense circuit 160 may generate a load current feedback signalV_(I-LOAD), which may be a DC voltage indicating the average magnitudeI_(AVE) of the load current I_(LOAD). The control circuit 150 mayreceive the load current feedback signal V_(I-LOAD) from the currentsense circuit 160. The control circuit 150 may adjust the drive controlsignals V_(DRIVE1), V_(DRIVE2) based on the load current feedback signalV_(I-LOAD) so that the magnitude of the load current I_(LOAD) may beadjusted towards a target load current I_(TRGT). For example, thecontrol circuit 150 may set initial operating parameters for the drivecontrol signals V_(DRIVE1), V_(DRIVE2) (e.g., the operating frequencyf_(OP) and/or the duty cycle DC_(INV)). The control circuit 150 mayreceive the load current feedback signal V_(I-LOAD) indicating theeffect of the drive control signals V_(DRIVE1), V_(DRIVE2). Based on theindication, the control circuit 150 may adjust the operating parametersof the drive control signals V_(DRIVE1), V_(DRIVE2) to thus adjust themagnitude of the load current I_(LOAD) towards a target load currentI_(TRGT) (e.g., using a control loop).

The load current I_(LOAD) may be the current that is conducted throughthe LED light source 120. The target load current I_(TRGT) may be thecurrent that the control circuit 150 aims to conduct through the LEDlight source 120 (e.g., based at least on the load current feedbacksignal V_(I-LOAD)). The load current I_(LOAD) may be approximately equalto the target load current I_(TRGT) but may not always match the targetload current I_(TRGT). This may be because, for example, the controlcircuit 150 may have specific levels of granularity in which it cancontrol the current conducted through the LED light source 120 (e.g.,due to inverter cycle lengths, etc.). A person skilled in the art willappreciate that the figures shown herein (e.g., FIG. 2) that illustratethe current conducted through an LED light source as a linear graph (atleast in parts) may represent the target load current I_(TRGT), sincethe load current I_(LOAD) itself may not be exactly equal to the targetload current I_(TRGT) and may not actually follow a true linear path.

The control circuit 150 may be coupled to the memory 170. The memory 170may store operational characteristics of the LED driver 100 (e.g., thetarget intensity L_(TRGT), the low-end intensity L_(LE), the high-endintensity L_(HE), etc.). The communication circuit 180 may be coupledto, for example, a wired communication link or a wireless communicationlink, such as a radio-frequency (RF) communication link or an infrared(IR) communication link. The control circuit 150 may be configured toupdate the target intensity L_(TRGT) of the LED light source 102 and/orthe operational characteristics stored in the memory 170 in response todigital messages received via the communication circuit 180. The LEDdriver 100 may be operable to receive a phase-control signal from adimmer switch for determining the target intensity L_(TRGT) for the LEDlight source 102. The power supply 190 may receive the rectified voltageV_(RECT) and generate a direct-current (DC) supply voltage V_(CC) forpowering the circuitry of the LED driver 100.

FIG. 2 is an example plot of the target load current I_(TRGT) as afunction of the target intensity L_(TRGT). As shown, a linearrelationship may exist between the target intensity L_(TRGT) and thetarget load current I_(TRGT). That is, to achieve a higher targetintensity, the control circuit 150 may increase the target load currentI_(TRGT) in proportion to the increase in the target intensity; toachieve a lower target intensity, the control circuit 150 may decreasethe target load current I_(TRGT) in proportion to the decrease in thetarget intensity. As the target load current I_(TRGT) is being adjusted,the magnitude of the load current I_(LOAD) may change accordingly. Theremay be limits, however, to how much the load current I_(LOAD) may beadjusted. For example, the load current I_(LOAD) may not be adjustedabove a maximum rated current I_(MAX) or below a minimum rated currentI_(MIN) (e.g., due to hardware limitations of the load regulationcircuit 140 and/or the control circuit 150). Thus, the control circuitmay be configured to adjust the target load current I_(TRGT) between themaximum rated current I_(MAX) and the minimum rated current I_(MIN) sothat the magnitude of the load current I_(LOAD) may fall into in thesame range. The maximum rated current I_(MAX) may correspond to thehigh-end intensity L_(HE) (e.g., approximately 100%). The minimum ratedcurrent I_(MIN) may correspond to a transition intensity L_(TRAN) (e.g.,approximately 5% of the maximum intensity). Between the high-endintensity L_(HE) and the transition intensity L_(TRAN), the controlcircuit 150 may operate the load regulation circuit 140 in a normal modein which an average magnitude I_(AVE) of the load current I_(LOAD) maybe controlled to be equal to (e.g., approximately equal to) the targetload current I_(TRGT). During the normal mode, the control circuit 150may adjust the average magnitude I_(AVE) of the load current I_(LOAD) tothe target load current I_(TRGT) in response to the load currentfeedback signal V_(I-LOAD) (e.g., using closed loop control), forexample. The control circuit 150 may apply various control techniquesduring the normal mode including, for example, a pulse-width modulationtechnique or a constant current reduction technique.

To adjust the average magnitude I_(AVE) of the load current I_(LOAD) tobelow the minimum rated current I_(MIN) (and to thus adjust the targetintensity L_(TRGT) below the transition intensity L_(TRAN)), the controlcircuit 150 may be configured to operate the load regulation circuit 140in a burst mode. The burst mode may be characterized by a burstoperating period that includes an active state period and an inactivestate period. During the active state period, the control circuit 150may be configured to regulate the load current I_(LOAD) in ways similarto those in the normal mode. During the inactive state period, thecontrol circuit 150 may be configured to stop regulating the loadcurrent LOAD (e.g., to allow the load current I_(LOAD) to drop toapproximately zero). Although the active state and inactive stateperiods are described herein in association with the burst mode, aperson skilled in the art will understand that the normal mode may alsobe characterized by an operating period that includes the active stateperiod and the inactive state period, e.g., with both periods heldconstant and the inactive state period held at approximately zero.Examples of a load control device capable of operating in a burst modeand a normal mode are described in greater detail in commonly-assignedU.S. Pat. No. 9,247,608, issued Jan. 26, 2016, entitled LOAD CONTROLDEVICE FOR A LIGHT-EMITTING DIODE LIGHT SOURCE, the entire disclosure ofwhich is hereby incorporated by reference.

The ratio of the active state period to the burst operating period,e.g., T_(ACTIVE)/T_(BURST), may represent a burst duty cycle DC_(BURST).The burst duty cycle DC_(BURST) may be controlled, for example, betweena maximum duty cycle DC_(MAX) (e.g., approximately 100%) and a minimumduty cycle DC_(MIN) (e.g., approximately 20%). The load current I_(LOAD)may be adjusted towards the target current I_(TRGT) (e.g., the minimumrated current I_(MIN)) during the active state period of the burst mode.Setting the burst duty cycle DC_(BURST) to a value less than the maximumduty cycle DC_(MAX) may reduce the average magnitude I_(AVE) of the loadcurrent I_(LOAD) to below the minimum rated current I_(MIN).

FIG. 3 is an example plot of a burst duty cycle DC_(BURST) (e.g., anideal burst duty cycle DC_(BURST-IDEAL)) as a function of the targetintensity L_(TRGT). As described herein, when the target intensityL_(TRGT) is between the high-end intensity L_(HE) (e.g., approximately100%) and a transition intensity L_(TRAN) (e.g., approximately 5% of themaximum intensity), the control circuit 150 may be configured to operatethe load regulation circuit 140 in the normal mode, e.g., by setting theburst duty cycle DC_(BURST) to a maximum duty cycle DC_(MAX) orapproximately 100%. To adjust the target intensity L_(TRGT) below thetransition intensity L_(TRAN), the control circuit 150 may be configuredto operate the load regulation circuit 140 in the burst mode, e.g., byadjusting the burst duty cycle DC_(BURST) between the maximum duty cycleDC_(MAX) (e.g., approximately 100%) and a minimum duty cycle DC_(MIN)(e.g., approximately 20%). In the burst mode, a peak magnitude I_(PK) ofthe load current I_(LOAD) may be equal to the target current I_(TRGT)(e.g., the minimum rated current I_(MIN)) during an active state periodof the burst mode.

With reference to FIG. 3, the burst duty cycle DC_(BURST) may refer toan ideal burst duty cycle DC_(BURST-IDEAL), which may include an integerportion DC_(BURST-INTEGER) and/or a fractional portionDC_(BURST-FRACTIONAL). The integer portion DC_(BURST-INTEGER) may becharacterized by the percentage of the ideal burst duty cycleDC_(BURST-IDEAL) that includes complete inverter cycles (i.e., aninteger value of inverter cycles). The fractional portionDC_(BURST-FRACTIONAL) may be characterized by the percentage of theideal burst duty cycle DC_(BURST-IDEAL) that includes a fraction of aninverter cycle. In at least some cases, the control circuit 150 (e.g.,via the load regulation circuit 140) may be configured to adjust thenumber of inverter cycles by an integer number (e.g., byDC_(BURST-INTEGER)) and not a fractional amount (e.g.,DC_(BURST-FRACTIONAL)). Therefore, although the example plot of FIG. 3illustrates an ideal curve showing continuous adjustment of the idealburst duty cycle DC_(BURST-IDEAL) from a maximum duty cycle DC_(MAX) toa minimum duty cycle DC_(MIN), unless defined differently, burst dutycycle DC_(BURST) may refer to the integer portion DC_(BURST-INTEGER) ofthe ideal burst duty cycle DC_(BURST-IDEAL) (e.g., if the controlcircuit 150 is not be configured to operate the burst duty cycleDC_(BURST) at fractional amounts).

FIG. 4 is an example state diagram illustrating the operation of theload regulation circuit 140 in the burst mode. During the burst mode,the control circuit 150 may periodically control the load regulationcircuit 140 into an active state and an inactive state, e.g., independence upon a burst duty cycle DC_(BURST) and a burst mode periodT_(BURST) (e.g., approximately 4.4 milliseconds). For example, theactive state period (T_(ACTIVE)) may be equal to the burst duty cycle(DC_(BURST)) times the burst mode period (T_(BURST)) and the inactivestate period (T_(INACTIVE)) may be equal to one minus the burst dutycycle (DC_(BURST)) times the burst mode period (T_(BURST)). That is,T_(ACTIVE)=DC_(BURST)−T_(BURST) and T_(INACTIVE)(1−DC_(BURST))−T_(BURST).

In the active state of the burst mode, the control circuit 150 may beconfigured to generate the drive control signals V_(DRIVE1), V_(DRIVE2).The control circuit 150 may be further configured to adjust theoperating frequency f_(OP) and/or the duty cycle DC_(INV) (e.g., an ontime T_(ON)) of the drive control signals V_(DRIVE1), V_(DRIVE2) inorder to adjust the magnitude of the load current I_(LOAD). The controlcircuit 150 may be configured to make the adjustments using closed loopcontrol. For example, in the active state of the burst mode, the controlcircuit 150 may generate the drive signals V_(DRIVE1), V_(DRIVE2) toadjust the magnitude of the load current I_(LOAD) to be equal to atarget load current I_(TRGT) (e.g., the minimum rated current I_(MIN))in response to the load current feedback signal V_(I-LOAD).

In the inactive state of the burst mode, the control circuit 150 may letthe magnitude of the load current I_(LOAD) drop to approximately zeroamps, e.g., by freezing the control loop and/or not generating the drivecontrol signals V_(DRIVE1), V_(DRIVE2). While the control loop is frozen(e.g., in the inactive state), the control circuit 150 may stopresponding to the load current feedback signal V_(I-LOAD) (e.g., thecontrol circuit 150 may not adjust the values of the operating frequencyf_(OP) and/or the duty cycle DC_(INV) in response to the feedbacksignal). The control circuit 150 may store the present duty cycleDC_(INV) (e.g., the present on time T_(ON)) of the drive control signalsV_(DRIVE1), V_(DRIVE2) in the memory 170 prior to (e.g., immediatelyprior to) freezing the control loop. When the control loop is unfrozen(e.g., when the control circuit 150 enters the active state), thecontrol circuit 150 may resuming generating the drive control signalsV_(DRIVE1), V_(DRIVE2) using the operating frequency f_(OP) and/or theduty cycle DC_(INV) from the previous active state.

The control circuit 150 may be configured to adjust the burst duty cycleDC_(BURST) using an open loop control. For example, the control circuit150 may be configured to adjust the burst duty cycle DC_(BURST) as afunction of the target intensity L_(TRGT) when the target intensityL_(TRGT) is below the transition intensity L_(TRAN). For example, thecontrol circuit 150 may be configured to linearly decrease the burstduty cycle DC_(BURST) as the target intensity L_(TRGT) is decreasedbelow the transition intensity L_(TRAN) (e.g., as shown in FIG. 3),while the target load current I_(TRGT) is held constant at the minimumrated current I_(MIN) (e.g., as shown in FIG. 2). Since the controlcircuit 150 changes between the active state and the inactive state independence upon the burst duty cycle DC_(BURST) and the burst modeperiod T_(BURST) (e.g., as shown in the state diagram of FIG. 4), theaverage magnitude I_(AVE) of the load current I_(LOAD) may be a functionof the burst duty cycle DC_(BURST) (e.g., I_(AVE)=DC_(BURST)−I_(MIN)).During the burst mode, the peak magnitude I_(PK) of the load currentI_(LOAD) may be equal to the minimum rated current I_(MIN), but theaverage magnitude I_(AVE) of the load current I_(LOAD) may be less thanthe minimum rated current I_(MIN).

FIG. 5 is a simplified schematic diagram of a forward converter 240 anda current sense circuit 260 of an LED driver (e.g., the LED driver 100shown in FIG. 1). The forward converter 240 may be an example of theload regulation circuit 140 of the LED driver 100 shown in FIG. 1. Thecurrent sense circuit 260 may be an example of the current sense circuit160 of the LED driver 100 shown in FIG. 1.

The forward converter 240 may comprise a half-bridge inverter circuithaving two field effect transistors (FETs) Q210, Q212 for generating ahigh-frequency inverter voltage V_(INV) from the bus voltage V_(BUS).The FETs Q210, Q212 may be rendered conductive and non-conductive inresponse to the drive control signals V_(DRIVE1), V_(DRIVE2). The drivecontrol signals V_(DRIVE1), V_(DRIVE2) may be received from the controlcircuit 150. The drive control signals V_(DRIVE1), V_(DRIVE2) may becoupled to the gates of the respective FETs Q210, Q212 via a gate drivecircuit 214 (e.g., which may comprise part number L6382DTR, manufacturedby ST Microelectronics). The control circuit 150 may be configured togenerate the inverter voltage V_(INV) at an operating frequency f_(OP)(e.g., approximately 60-65 kHz) and thus an operating period T_(OP). Thecontrol circuit 150 may be configured to adjust the operating frequencyf_(OP) under certain operating conditions. The control circuit 150 maybe configured to adjust a duty cycle DC_(INV) of the inverter voltageV_(INV) to control the intensity of an LED light source 202 towards thetarget intensity L_(TRGT).

In a normal mode of operation, when the target intensity L_(TRGT) of theLED light source 202 is between the high-end intensity L_(HE) and thetransition intensity L_(TRAN), the control circuit 150 may adjust theduty cycle DC_(INV) of the inverter voltage V_(INV) to adjust themagnitude (e.g., the average magnitude I_(AVE)) of the load currentI_(LOAD) towards the target load current I_(TRGT). As described herein,the magnitude of the load current I_(LOAD) may vary between the maximumrated current I_(MAX) and the minimum rated current I_(MIN) (e.g., asshown in FIG. 2). At the minimum rated current I_(MIN) and/or thetransition intensity L_(TRAN), the inverter voltage V_(INV) may becharacterized by a transition (e.g., from a normal mode to a burst mode)operating frequency f_(OP-T), a transition operating period T_(OP-T),and a transition duty cycle DC_(INV-T).

When the target intensity L_(TRGT) of the LED light source 202 is belowthe transition intensity L_(TRAN), the control circuit 150 may beconfigured to operate the forward converter 240 in a burst mode ofoperation. In addition to or in lieu of using target intensity as athreshold for determining when to operate in the burst mode, the controlcircuit 150 may use power (e.g., a transition power) and/or current(e.g., a transition current) as the threshold. In the burst mode ofoperation, the control circuit 150 may be configured to switch theforward converter 240 between an active state (e.g., in which thecontrol circuit 150 may actively generate the drive control signalsV_(DRIVE1), V_(DRIVE2) to regulate the peak magnitude I_(PK) of the loadcurrent I_(LOAD) to be equal to the minimum rated current I_(MIN)) andan inactive state (e.g., in which the control circuit 150 may freeze thecontrol loop and does not generate the drive control signals V_(DRIVE1),V_(DRIVE2)). FIG. 4 shows a state diagram illustrating the transmissionbetween the two states. The control circuit 150 may change the forwardconverter 240 between the active state and the inactive state independence upon a burst duty cycle DC_(BURST) and a burst mode periodT_(BURST) (e.g., as shown in FIG. 4). The control circuit 150 may adjustthe burst duty cycle DC_(BURST) as a function of the target intensityL_(TRGT), which is below the transition intensity L_(TRAN) (e.g., asshown in FIG. 3). In the active state of the burst mode (as well as inthe normal mode), the forward converter 240 may be characterized by aturn-on time T_(TURN-ON) and a turn-off time T_(TURN-OFF). The turn-ontime T_(TURN-ON) may be a time period from when the drive controlsignals V_(DRIVE1), V_(DRIVE2) are driven until the respective FET Q210,Q212 is rendered conductive. The turn-off time T_(TURN-OFF) may be atime period from when the drive control signals V_(DRIVE1), V_(DRIVE2)are driven until the respective FET Q210, Q212 is renderednon-conductive.

The inverter voltage V_(INV) may be coupled to the primary winding of atransformer 220 through a DC-blocking capacitor C216 (e.g., which mayhave a capacitance of approximately 0.047 μF). A primary voltage V_(PRI)may be generated across the primary winding. The transformer 220 may becharacterized by a turns ratio n_(TURNS) (e.g., N₁/N₂), which may beapproximately 115:29. A sense voltage V_(SENSE) may be generated acrossa sense resistor R222, which may be coupled in series with the primarywinding of the transformer 220. The FETs Q210, Q212 and the primarywinding of the transformer 220 may be characterized by parasiticcapacitances C_(P1), C_(P2), C_(P3), respectively. The secondary windingof the transformer 220 may generate a secondary voltage. The secondaryvoltage may be coupled to the AC terminals of a full-wave dioderectifier bridge 224 for rectifying the secondary voltage generatedacross the secondary winding. The positive DC terminal of the rectifierbridge 224 may be coupled to the LED light source 202 through an outputenergy-storage inductor L226 (e.g., which may have an inductance ofapproximately 10 mH). The load voltage V_(LOAD) may be generated acrossan output capacitor C228 (e.g., which may have a capacitance ofapproximately 3 μF).

The current sense circuit 260 may comprise an averaging circuit forproducing the load current feedback signal V_(I-LOAD). The averagingcircuit may comprise a low-pass filter comprising a capacitor C230(e.g., which may have a capacitance of approximately 0.066 uF) and aresistor R232 (e.g., which may have a resistance of approximately 3.32kΩ). The low-pass filter may receive the sense voltage V_(SENSE) via aresistor R234 (e.g., which may have a resistance of approximately 1 kΩ).The current sense circuit 160 may comprise a transistor Q236 (e.g., aFET as shown in FIG. 5) coupled between the junction of the resistorsR232, R234 and circuit common. The gate of the transistor Q236 may becoupled to circuit common through a resistor R238 (e.g., which may havea resistance of approximately 22 kΩ). The gate of the transistor Q236may receive the signal-chopper control signal V_(CHOP) from the controlcircuit 150. An example of the current sense circuit 260 is described ingreater detail in commonly-assigned U.S. patent application Ser. No.13/834,153, filed Mar. 15, 2013, entitled FORWARD CONVERTER HAVING APRIMARY-SIDE CURRENT SENSE CIRCUIT, the entire disclosure of which ishereby incorporated by reference.

FIG. 6 is an example diagram illustrating a magnetic core set 290 of anenergy-storage inductor (e.g., the output energy-storage inductor L226of the forward converter 240 shown in FIG. 5). The magnetic core set 290may comprise two E-cores 292A, 292B, and may comprise part numberPC40EE16-Z, manufactured by TDK Corporation. The E-cores 292A, 292B maycomprise respective outer legs 294A, 294B and inner legs 296A, 296B. Theinner legs 296A, 296B may be characterized by a width w_(LEG) (e.g.,approximately 4 mm). The inner leg 296A of the first E-core 292A maycomprise a partial gap 298A (e.g., the magnetic core set 290 may bepartially-gapped), such that the inner legs 296A, 296B may be spacedapart by a gap distance d_(GAP) (e.g., approximately 0.5 mm). Thepartial gap 298A may extend for a gap width w_(GAP) (e.g., approximately2.8 mm) such that the partial gap 298A may extend for approximately 70%of the leg width w_(LEG) of the inner leg 296A. Either or both of theinner legs 296A, 296B may comprise partial gaps. The partially-gappedmagnetic core set 290 (e.g., as shown in FIG. 6) may allow the outputenergy-storage inductor L226 of the forward converter 240 (e.g., shownin FIG. 5) to maintain continuous current at low load conditions (e.g.,near the low-end intensity L_(LE)).

FIG. 7 shows example waveforms illustrating the operation of a forwardconverter (e.g., the forward converter 240) and a current sense circuit(e.g., the current sense circuit 260). The forward converter 240 maygenerate the waveforms shown in FIG. 7, for example, when operating inthe normal mode and in the active state of the burst mode as describedherein. As shown in FIG. 7, a control circuit (e.g., the control circuit150) may drive the respective drive control signals V_(DRIVE1),V_(DRIVE2) high to approximately the supply voltage V_(CC) to render therespective FETs Q210, Q212 conductive for an on time T_(ON). The FETsQ210, Q212 may be rendered conductive at different times. When thehigh-side FET Q210 is conductive, the primary winding of the transformer220 may conduct a primary current I_(PRI) to circuit common through thecapacitor C216 and sense resistor R222. After (e.g., immediately after)the high-side FET Q210 is rendered conductive (at time t₁ in FIG. 7),the primary current I_(PRI) may conduct a short high-magnitude pulse ofcurrent due to the parasitic capacitance C_(P3) of the transformer 220as shown in FIG. 7. While the high-side FET Q210 is conductive, thecapacitor C216 may charge, such that a voltage having a magnitude ofapproximately half of the magnitude of the bus voltage V_(BUS) may bedeveloped across the capacitor. The magnitude of the primary voltageV_(PRI) across the primary winding of the transformer 220 may be equalto approximately half of the magnitude of the bus voltage V_(BUS) (e.g.,V_(BUS)/2). When the low-side FET Q212 is conductive, the primarywinding of the transformer 220 may conduct the primary current I_(PRI)in an opposite direction and the capacitor C216 may be coupled acrossthe primary winding, such that the primary voltage V_(PRI) may have anegative polarity with a magnitude equal to approximately half of themagnitude of the bus voltage V_(BUS).

When either of the high-side and low-side FETs Q210, Q212 areconductive, the magnitude of an output inductor current I_(L) conductedby the output inductor L226 and/or the magnitude of the load voltageV_(LOAD) across the LED light source 202 may increase with respect totime. The magnitude of the primary current I_(PRI) may increase withrespect to time while the FETs Q210, Q212 are conductive (e.g., after aninitial current spike). When the FETs Q210, Q212 are non-conductive, theoutput inductor current I_(L) and the load voltage V_(LOAD) may decreasein magnitude with respective to time. The output inductor current I_(L)may be characterized by a peak magnitude I_(L-PK) and an averagemagnitude I_(L-AVG), for example, as shown in FIG. 7. The controlcircuit 150 may increase and/or decrease the on times T_(ON) of thedrive control signals V_(DRIVE1), V_(DRIVE2) (e.g., and the duty cycleDC_(INV) of the inverter voltage V_(INV)) to respectively increase anddecrease the average magnitude I_(L-AVG) of the output inductor currentI_(L), and thus respectively increase and decrease the intensity of theLED light source 202.

When the FETs Q210, Q212 are rendered non-conductive, the magnitude ofthe primary current I_(PRI) may drop toward zero amps (e.g., as shown attime t₂ in FIG. 7 when the high-side FET Q210 is renderednon-conductive). A magnetizing current I_(MAG) may continue to flowthrough the primary winding of the transformer 220, for example, due tothe magnetizing inductance L_(MAG) of the transformer. When the targetintensity L_(TRGT) of the LED light source 102 is near the low-endintensity L_(LE), the magnitude of the primary current I_(PRI) mayoscillate after either of the FETs Q210, Q212 is renderednon-conductive. The oscillation may be caused by the parasiticcapacitances C_(P1), C_(P2) of the FETs, the parasitic capacitanceC_(P3) of the primary winding of the transformer 220, and/or any otherparasitic capacitances of the circuit (e.g., such as the parasiticcapacitances of the printed circuit board on which the forward converter240 is mounted).

The real component of the primary current I_(PRI) may indicate themagnitude of the secondary current I_(SEC) and thus the intensity of theLED light source 202. The magnetizing current I_(MAG) (e.g., thereactive component of the primary current I_(PRI)) may flow through thesense resistor R222. When the high-side FET Q210 is conductive, themagnetizing current I_(MAG) may change from a negative polarity to apositive polarity. When the low-side FET Q210 is conductive, themagnetizing current I_(MAG) may change from a positive polarity to anegative polarity. When the magnitude of the primary voltage V_(PRI) iszero volts, the magnetizing current I_(MAG) may remain constant, forexample, as shown in FIG. 7. The magnetizing current I_(MAG) may have amaximum magnitude defined by the following equation:

$I_{{MAG}\text{-}{MA}\; X} = \frac{V_{BUS} \cdot T_{HC}}{4 \cdot L_{MAG}}$

, where T_(HC) may be the half-cycle period of the inverter voltageV_(INV), e.g., T_(HC)=T_(OP)/2. As shown in FIG. 7, the areas 250, 252may be approximately equal, such that the average value of the magnitudeof the magnetizing current I_(MAG) may be zero during the period of timewhen the magnitude of the primary voltage V_(PRI) is greater thanapproximately zero volts (e.g., during the on time T_(ON) as shown inFIG. 7).

The current sense circuit 260 may determine an average of the primarycurrent I_(PRI) during the positive cycles of the inverter voltageV_(INV), e.g., when the high-side FET Q210 is conductive. As describedherein, the high-side FET Q210 may be conductive during the on timeT_(ON). The load current feedback signal V_(I-LOAD), which may begenerated by the current sense circuit 260, may have a DC magnitude thatis the average value of the primary current I_(PRI) (e.g., when thehigh-side FET Q210 is conductive). Because the average value of themagnitude of the magnetizing current I_(MAG) may be approximately zeroduring the period of time that the high-side FET Q210 is conductive(e.g., during the on time T_(ON)), the load current feedback signalV_(I-LOAD) generated by the current sense circuit may indicate the realcomponent (e.g., only the real component) of the primary current I_(PRI)(e.g., during the on time T_(ON)).

When the high-side FET Q210 is rendered conductive, the control circuit150 may drive the signal-chopper control signal V_(CHOP) low towardscircuit common to render the transistor Q236 of the current sensecircuit 260 non-conductive for a signal-chopper time T_(CHOP). Thesignal-chopper time T_(CHOP) may be approximately equal to the on timeT_(ON) of the high-side FET Q210, for example, as shown in FIG. 7. Thecapacitor C230 may charge from the sense voltage V_(SENSE) through theresistors R232, R234 while the signal-chopper control signal V_(CHOP) islow. The magnitude of the load current feedback signal V_(I-LOAD) may bethe average value of the primary current I_(PRI) and may indicate thereal component of the primary current during the time when the high-sideFET Q210 is conductive. When the high-side FET Q210 is not conductive,the control circuit 150 may drive the signal-chopper control signalV_(CHOP) high to render the transistor Q236 conductive. Accordingly, thecontrol circuit 150 may be able to determine the average magnitude ofthe load current I_(LOAD) from the magnitude of the load currentfeedback signal V_(I-LOAD), at least partially because the effects ofthe magnetizing current I_(MAG) and the oscillations of the primarycurrent I_(PRI) on the magnitude of the load current feedback signalV_(I-LOAD) may be reduced or eliminated.

As the target intensity L_(TRGT) of the LED light source 202 isdecreased towards the low-end intensity L_(LE) and/or the on timesT_(ON) of the drive control signals V_(DRIVE1), V_(DRIVE2) get smaller,the parasitic of the load regulation circuit 140 (e.g., the parasiticcapacitances C_(P1), C_(P2) of the FETs Q210, Q212, the parasiticcapacitance C_(P3) of the primary winding of the transformer 220, and/orother parasitic capacitances of the circuit) may cause the magnitude ofthe primary voltage V_(PRI) to slowly decrease towards zero volts afterthe FETs Q210, Q212 are rendered non-conductive.

FIG. 8 shows example waveforms illustrating the operation of a forwardconverter and a current sense circuit (e.g., the forward converter 240and the current sense circuit 260) when the target intensity L_(TRGT) isnear the low-end intensity L_(LE), and when the forward converter 240 isoperating in the normal mode and the active state of the burst mode. Thegradual drop off in the magnitude of the primary voltage V_(PRI) mayallow the primary winding of the transformer 220 to continue to conductthe primary current I_(PRI), such that the transformer 220 may continueto deliver power to the secondary winding after the FETs Q210, Q212 arerendered non-conductive, for example, as shown in FIG. 8. Themagnetizing current I_(MAG) may continue to increase in magnitude afterthe on time T_(ON) of the drive control signal V_(DRIVE1) (e.g., and/orthe drive control signal V_(DRIVE2)). The control circuit 150 mayincrease the signal-chopper time T_(CHOP) to be greater than the on timeT_(ON). For example, the control circuit 150 may increase thesignal-chopper time T_(CHOP) (e.g., during which the signal-choppercontrol signal V_(CHOP) is low) by an offset time T_(OS) when the targetintensity L_(TRGT) of the LED light source 202 is near the low-endintensity L_(LE).

FIG. 9 shows example waveforms illustrating the operation of a forwardconverter (e.g., the forward converter 240 shown in FIG. 5) whenoperating in a burst mode. The inverter circuit of the forward converter240 may generate the inverter voltage V_(INV) during an active state(e.g., for the duration of an active state period T_(ACTIVE)). A purposeof the inverter voltage V_(INV) may be to regulate the magnitude of theload current I_(LOAD) to the minimum rated current I_(MIN) during theactive state period. During an inactive state period, the invertervoltage V_(INV) may be reduced to zero (e.g., not generated). Theforward converter may enter the active state on a periodic basis with aninterval approximately equal to a burst mode period T_(BURST) (e.g.,approximately 4.4 milliseconds). The active state period T_(ACTIVE) andinactive state period T_(INACTIVE) may be characterized by durationsthat are dependent upon a burst duty cycle DC_(BURST), e.g.,T_(ACTIVE)=DC_(BURST)−T_(BURST) andT_(INACTIVE)=(1−DC_(BURST))−T_(BURST). The average magnitude I_(AVE) ofthe load current I_(LOAD) may be dependent on the burst duty cycleDC_(BURST). For example, the average magnitude I_(AVE) of the loadcurrent I_(LOAD) may be equal to the burst duty cycle DC_(BURST) timesthe load current I_(LOAD) (e.g., I_(AVE)=DC_(BURST)−I_(LOAD)). When theload current I_(LOAD) is equal to the minimum load current I_(MIN), theaverage magnitude I_(AVE) of the load current I_(LOAD) may be equal toI_(AVE)=DC_(BURST)−I_(MIN).

The burst duty cycle DC_(BURST) may be controlled to adjust the averagemagnitude I_(AVE) of the load current I_(LOAD). The burst duty cycleDC_(BURST) may be controlled in different ways. For example, the burstduty cycle DC_(BURST) may be controlled by holding the burst mode periodT_(BURST) constant and varying the length of the active state periodT_(ACTIVE). The burst duty cycle DC_(BURST) may also be controlled byholding the active state period T_(ACTIVE) constant and varying thelength of the inactive state period T_(INACTIVE) (and thus varying thelength of the burst mode period T_(BURST)). As the burst duty cycleDC_(BURST) is increased, the average magnitude I_(AVE) of the loadcurrent I_(LOAD) may increase. As the burst duty cycle DC_(BURST) isdecreased, the average magnitude I_(AVE) of the load current I_(LOAD)may decrease. The control circuit 150 may be configured to adjust theburst duty cycle DC_(BURST) using open loop control (e.g., in responseto the target intensity L_(TRGT)). The control circuit 150 may beconfigured to adjust the burst duty cycle DC_(BURST) using closed loopcontrol (e.g., in response to the load current feedback signalV_(I-LOAD)).

FIG. 10 shows a diagram of an example waveform 1000 illustrating theload current I_(LOAD) when a load regulation circuit (e.g., the loadregulation circuit 140) is operating in a burst mode, for example, asthe target intensity L_(TRGT) of a light source (e.g., the LED lightsource 202) is being increased (e.g., from the low-end intensityL_(LE)). A control circuit (e.g., the control circuit 150 of the LEDdriver 100 shown in FIG. 1 and/or the control circuit 150 controllingthe forward converter 240 and the current sense circuit 260 shown inFIG. 5) may adjust the length of the active state period T_(ACTIVE) ofthe burst mode period T_(BURST) by adjusting the burst duty cycleDC_(BURST). Adjusting the length of the active state period T_(ACTIVE)may adjust the average magnitude I_(AVE) of the load current I_(LOAD),and in turn the intensity of the light source.

The active state period T_(ACTIVE) of the load current I_(LOAD) may havea length that is dependent upon the length of an inverter cycle of theinverter circuit of the load regulation circuit (e.g., the operatingperiod T_(OP)). For example, the active state period T_(ACTIVE) maycomprise six inverter cycles, and as such, may have a length that isequal to the duration of the six inverter cycles. The control circuitmay adjust (e.g., increase or decrease) the length of the active stateperiods T_(ACTIVE) by adjusting the number of inverter cycles in theactive state period T_(ACTIVE). As such, the control circuit may adjustthe length of the active state periods T_(ACTIVE) by predeterminedincrements/decrements, e.g., with each increment/decrement correspondingto approximately the length of an inverter cycle (e.g., such as thetransition operating period T_(OP-T), which may be approximately 12.8microseconds). Since the average magnitude I_(AVE) of the load currentI_(LOAD) may depend on the active state period T_(ACTIVE), the averagemagnitude I_(AVE) may also be adjusted by a predeterminedincrement/decrement that corresponds to a change in the load currentI_(LOAD) resulting from the addition or removal of an inverter cycle peractive state period T_(ACTIVE).

FIG. 10 shows four burst mode periods T_(BURST) 1002, 1004, 1006, 1008with equivalent length. The first three burst mode periods T_(BURST)1002, 1004, 1006 may be characterized by equivalent active state periodsT_(ACTIVE1) (e.g., with the same number of inverter cycles) andequivalent inactive state periods T_(INACTIVE1). The fourth burst modeperiod T_(BURST) 1008 may be characterized by an active state periodT_(ACTIVE2) that is larger than the active state periods T_(ACTIVE1)(e.g., by one more inverter cycle), and an inactive state periodT_(INACTIVE2) that is smaller than the inactive state periodT_(INACTIVE1) (e.g., by one fewer inverter cycle). The larger activestate period T_(ACTIVE2) and smaller inactive state period T_(INACTIVE2)may result in a larger duty cycle and a corresponding larger averagemagnitude I_(AVE) of the load current I_(LOAD) (e.g., as shown duringburst mode period 1008). As the average magnitude I_(AVE) of the loadcurrent I_(LOAD) increases, the intensity of the light source mayincrease accordingly. Hence, as shown in FIG. 10, by adding invertercycles to or removing inverter cycles from the active state periodsT_(ACTIVE) while maintaining the length of the burst mode periodsT_(BURST), the control circuit may adjust the average magnitude I_(AVE)of the load current I_(LOAD). Such adjustments to only the active stateperiods T_(ACTIVE), however, may cause changes in the intensity of thelighting load that are perceptible to the user, e.g., when the targetintensity is equal to or below the transition intensity L_(TRAN).

FIG. 11 illustrates how the relative average light intensity of a lightsource may change as a function of the number N_(INV) of inverter cyclesincluded in an active state period T_(ACTIVE) if the control circuitonly adjusts the active state periods T_(ACTIVE) during the burst mode.As described herein, T_(ACTIVE) may be expressed asT_(ACTIVE)=N_(INV)−T_(OP-LE), wherein T_(OP-LE) may represent a low-endoperating period of the relevant inverter circuit. As shown in FIG. 11,if the control circuit adjusts the length of the active state periodsT_(ACTIVE) from four to five inverter cycles, the relative lightintensity may change by approximately 25%. If the control circuitadjusts the length of the active state periods T_(ACTIVE) from five tosix inverter cycles, the relative light intensity may change byapproximately 20%.

Fine tuning of the light level or light intensity of the lighting loadmay be achieved by configuring the control circuit to adjust (e.g.,increase or decrease) the length of the inactive state periodsT_(INACTIVE) in the burst mode. Adjustments to the length of theinactive state periods T_(INACTIVE) may be made between adjusting thelength of the active state periods T_(ACTIVE). Adjustments to the lengthof the inactive state periods T_(INACTIVE) may also be made whileadjusting the length of the active state periods T_(ACTIVE). Theadjustments to the inactive state periods T_(INACTIVE) may be made inone or more steps with respective adjustment amounts. The respectiveadjustment amounts may be substantially equal to or different from eachother. The respective adjustment amounts may be determined such that anadjustment made to the inactive state periods will cause a same orsmaller change to the light intensity (e.g., a smaller change relativeto a specific light intensity level) than an adjustment to active stateperiods (e.g., by one inverter cycle) would have caused had the inactivestate periods not been changed. In an example, one or more of therespective adjustment amounts made to the inactive state periods may besmaller than an adjustment amount made to the active state periods. Inan example, the respective adjustment amounts made to the inactive stateperiods may not be smaller than the adjustment amount made to the activestate periods, but the changes caused by the respective inactiveadjustment amounts to the relative light intensity may still be smallerthan the change caused by the active state adjustment amount. Thecontrol circuit may adjust the length of the inactive state periodsT_(INACTIVE) as a function of the target intensity L_(TRGT) of thelighting load.

FIG. 12 shows example waveforms 1210-1280 illustrating the load currentI_(LOAD) when a load regulation circuit (e.g., the load regulationcircuit 140) is controlled (e.g., by the control circuit 150) to operatein the burst mode. More specifically, the illustrated example shows thatthe control circuit may adjust the target intensity L_(TRGT) of thelight source (e.g., the LED light source 202) by first adjusting thelength of the inactive state periods and then adjusting the length ofthe active state periods. By using the control technique shown in FIG.12, the control circuit may accomplish fine dimming of the lightingload.

As shown in FIG. 12, the control circuit may control the load currentI_(LOAD) to have a default burst mode period T_(BURST-DEF) (e.g., asshown in waveform 1210). For example, the default burst mode periodT_(BURST-DEF) may be approximately 800 microseconds to correspond to afrequency of approximately 1.25 kHz. The inverter circuit comprised inthe load regulation circuit may be characterized by an operatingfrequency f_(OP-BURST) (e.g., approximately 25 kHz) and an operatingperiod T_(OP-BURST) (e.g., approximately 40 microseconds). The controlcircuit may adjust the length of the inactive state periods T_(INACTIVE)gradually, for example, between adjusting the length of the active stateperiods T_(ACTIVE). The adjustment to the length of the inactive stateperiods T_(INACTIVE) may be made in one or more steps (e.g., over one ormore adjacent or separate burst mode periods) with respective inactivestate adjustment amounts Δ_(INACTIVE). The respective inactive stateadjustment amounts may be substantially the same for each step or may bedifferent for different steps, so long as the adjustments may allow finetuning of the light intensity of the lighting load. For example, theinactive-state adjustment amount Δ_(INACTIVE) may be equal to apercentage (e.g., approximately 1%) of the default burst mode periodT_(BURST-DEF) (e.g., approximately 8 microseconds).

The control circuit may adjust the length of the inactive state periodsT_(INACTIVE) (e.g., by the inactive-state adjustment amount Δ_(INACTIVE)each time) while maintaining the length of the active state periodT_(ACTIVE) constant (as shown in waveforms 1210-1260 in FIG. 12). Whenthe length of the inactive state periods T_(INACTIVE) has been adjustedby a threshold amount (e.g., a maximum adjustment amountΔ_(INACTIVE-MAX), as shown in waveform 1260), the control circuit mayadjust the length of the active state periods T_(ACTIVE) by an activestate adjustment amount Δ_(ACTIVE) (e.g., by one additional invertercycle length) in a succeeding burst mode period, for example. Thecontrol circuit may adjust the length of the inactive state periods(e.g., in the same succeeding burst mode period) such that the length ofthe burst mode period T_(BURST) may revert back to that of the defaultburst mode period T_(BURST-DEF), and the length of the inactive stateperiods T_(INACTIVE) may be equal to the difference between the defaultburst mode period T_(BURST-DEF) and the present length of the activestate periods T_(ACTIVE) (as shown in waveform 1270 of FIG. 12). Thecontrol circuit may then go back to adjusting the length of the inactivestate periods T_(INACTIVE) as described herein until the length of theinactive state periods T_(INACTIVE) has once again been adjusted by themaximum adjustment amount Δ_(INACTIVE-MAX). At that point, the controlcircuit may adjust the length of the active state periods T_(ACTIVE)and/or the length of the inactive state periods T_(INACTIVE) such thatthe burst mode period T_(BURST) may again be adjusted back to thedefault burst mode period T_(BURST-DEF). Eventually, the burst dutycycle DC_(BURST) may reach approximately 100% (e.g., as shown inwaveform 1280) and the light intensity of the lighting load may reachthe transition intensity L_(TRAN). Beyond that point, the controlcircuit may begin adjusting the target load current I_(TRGT) in thenormal mode (e.g., via PWM or CCR).

As described herein, the user's eyes may be more sensitive to changes inthe relative light level of the lighting load when the light level islow (e.g., below to the transition intensity L_(TRAN)). The maximumadjustment amount Δ_(INACTIVE-MAX) for the inactive state periodsT_(INACTIVE) may be sized to reduce perceptible changes in the relativelight level of the lighting load. For example, if the lengths of theactive state periods T_(ACTIVE) and the inactive state periodsT_(INACTIVE) are both adjusted (e.g., between waveforms 1260 and 1270 inFIG. 12), a properly sized maximum adjustment amount Δ_(INACTIVE-MAX)may enable a smooth transition from a current intensity level into thenext intensity level. The maximum adjustment amount Δ_(INACTIVE-MAX) maybe determined as a function of the present length of the active stateperiod T_(ACTIVE) (e.g., the number of inverter cycles included in theactive state period T_(ACTIVE)). The determination may be made bycalculating a value for the maximum adjustment amount Δ_(INACTIVE-MAX)in real-time or by retrieving a predetermined value from memory (e.g.,from a lookup table). In an example, when the active state periodT_(ACTIVE) presently includes four inverter cycles, the maximumadjustment amount Δ_(INACTIVE-MAX) may be approximately equal to thechange in the relative light level when the length of the active stateperiod T_(ACTIVE) changes from four to five inverter cycles (e.g., 25%as shown in FIG. 11). In another example, the maximum adjustment amountΔ_(INACTIVE-MAX) may be approximately equal to the burst operatingperiod T_(OP-BURST) of the inverter circuit (e.g., approximately 40microseconds). The control circuit may store the value of the maximumadjustment amount Δ_(INACTIVE-MAX) in memory (e.g., in a lookup table)

FIG. 13 shows two example plot relationships depicting how a targetlight intensity of the lighting load may change in accordance withchanges in the lengths of the active and inactive state periods when aload regulation circuit (e.g., the load regulation circuit 140) iscontrolled (e.g., by the control circuit 150) to operate in the burstmode. Plot 1300 shows an example relationship between the length of theinactive state period T_(INACTIVE) and the target intensity L_(TRGT) ofthe lighting load. Plot 1310 shows an example relationship between thelength of the active state period T_(ACTIVE) and the target intensityL_(TRGT) of the lighting load. The length of the active state periodT_(ACTIVE) may be expressed in time terms or in terms of the number ofinverter cycles N_(INV) included in the active state period T_(ACTIVE),for example.

As described herein, the control circuit (e.g., the control circuit 150of the LED driver 100 shown in FIG. 1 and/or the control circuit 150controlling the forward converter 240 and the current sense circuit 260shown in FIG. 5) may determine the magnitude of the target load currentI_(TRGT) and/or the burst duty cycle DC_(BURST) based on the targetintensity L_(TRGT). The control circuit may determine the targetintensity L_(TRGT), for example, via a digital message received via thecommunication circuit 180, via a phase-control signal received from adimmer switch, and/or the like. The target intensity L_(TRGT) may beconstant or may be changing (e.g., fading) from one intensity level toanother. The control circuit may determine the length of the activestate period T_(ACTIVE) based on the target intensity L_(TRGT). Afterdetermining the length of the active state period T_(ACTIVE), thecontrol circuit may determine the length of the inactive state periodT_(INACTIVE) that may be used with the present active state periodT_(ACTIVE) such that the light source may be driven to the targetintensity L_(TRGT). The control circuit may determine the lengths of theactive state period T_(ACTIVE) and/or the inactive state periodT_(INACTIVE) by calculating the values in real-time and/or retrievingthe values from memory (e.g., via a lookup table or the like).

Referring to FIG. 13, if the control circuit determines that the targetintensity L_(TRGT) falls within the range 1321, then the control circuitmay determine to set the burst mode period to a default burst modeperiod (e.g., such as T_(BURST-DEF), which may be approximately 800microseconds) and the active state period T_(ACTIVE) to a minimum activestate period T_(ACTIVE-MIN) (e.g., including four inverter cycles). Thecontrol circuit may determine to set the inactive state periodT_(INACTIVE) according to the profile 1341, which may range from amaximum inactive state period T_(INACTIVE-MAX) to a minimum inactivestate period T_(MIN1). The maximum inactive state periodT_(INACTIVE-MAX) may be determined based on the length of the presentburst operating period (e.g., the default burst mode periodT_(BURST-DEF)) and/or the length of the present active state periodT_(ACTIVE-MIN). The minimum inactive state period T_(MIN1) may bedetermined based on the maximum inactive state adjustment amountΔ_(INACTIVE-MAX), which may in turn be dependent upon the length of thepresent active state period T_(ACTIVE-MIN). The gradient of the profile1341 may be determined based on the size of an inactive state adjustmentstep (e.g., such as the inactive state adjustment amount Δ_(INACTIVE)),which, may be equal to a percentage (e.g., approximately 1%) of thedefault burst mode period T_(BURST-DEF), for example. As noted herein,the control circuit may determine the lengths of the active state periodT_(ACTIVE) and/or the inactive state period T_(INACTIVE) by calculatingthe values in real-time and/or retrieving the values from memory.

If the control circuit determines that the target intensity L_(TRGT)falls within the range 1322, then the control circuit may determine toset the active state period T_(ACTIVE) to 1332. The active state period1332 may be greater than the minimum active state period T_(ACTIVE-MIN).For example, the active state period 1332 may include one more invertercycle than the minimum active state period T_(ACTIVE-MIN). The controlcircuit may determine to set the inactive state period T_(INACTIVE)according to the profile 1342. In an example, the starting point of theprofile 1342 may be dependent upon the length of the present burst cycleperiod (e.g., the default burst cycle period T_(BURST-DEF)) and thelength of the present active state period 1332. The ending point of theprofile 1342 may be dependent upon the maximum inactive state adjustmentamount Δ_(INACTIVE-MAX), which may in turn be dependent upon the lengthof the present active state period 1332. The gradient of the profile1342 may be determined based on the size of an inactive-state adjustmentstep (e.g., such as the inactive-state adjustment amount Δ_(INACTIVE)),which, as noted herein, may be equal to a percentage (e.g.,approximately 1%) of the default burst mode period T_(BURST-DEF).Similarly, if the control circuit determines that the target intensityL_(TRGT) falls within one of the target intensity ranges 1323-1327, thenthe control circuit may determine to set the active state periodT_(ACTIVE) to one of 1333-1337 and determine to set the inactive stateperiod T_(INACTIVE) according to one of the profiles 1343-1347,respectively.

The profiles 1341-1347 may be linear or non-linear, and may becontinuous (e.g., as shown in FIG. 13) or comprise discrete steps. Theminimum inactive state periods for the profiles 1341-1347 may bedependent upon the present maximum adjustment amount Δ_(INACTIVE-MAX),which may in turn be dependent upon the length of the respective activestate period T_(ACTIVE). The maximum adjustment amount Δ_(INACTIVE-MAX)of the inactive state period T_(INACTIVE) may be sized to reduceperceptible changes in the relative light level of the lighting load. Inan example, the profiles 1341-1347 may be configured such that when thelengths of the active state period T_(ACTIVE) and the inactive stateperiod T_(INACTIVE) are both adjusted (e.g., between waveforms 1260 and1270 as shown in FIG. 12), the waveform characterized by the greatertarget intensity may generate a greater light output of the lightingload. In such an example, there may be slight steps up in the actuallight output of the lighting load when the lengths of the active stateperiod T_(ACTIVE) and the inactive state period T_(INACTIVE) are bothadjusted (e.g., between waveforms 1260 and 1270 as shown in FIG. 12).

The graphs 1300, 1310 may represent a portion of the target intensityrange between the low-end intensity L_(LE) and the transition intensityL_(TRAN) or the entire target intensity range between the low-endintensity L_(LE) and the transition intensity L_(TRAN). More or lessthan seven active state periods T_(ACTIVE) (e.g., T_(ACTIVE-MIN) through1337) may be provided between the low-end intensity L_(LE) and thetransition intensity L_(TRAN).

FIG. 14 illustrates an example target intensity procedure 1400 that maybe executed by the control circuit described herein (e.g., the controlcircuit 150 of the LED driver 100 shown in FIG. 1 and/or the controlcircuit 150 controlling the forward converter 240 and the current sensecircuit 260 shown in FIG. 5). For example, the target intensityprocedure 1400 may be executed when the target intensity L_(TRGT) isadjusted at 1410 (e.g., in response to digital messages received via thecommunication circuit 180). The control circuit may determine if it isoperating the load regulation circuit in the burst mode at 1412 (e.g.,the target intensity L_(TRGT) is between the low-end intensity L_(LE)and the transition intensity L_(TRAN), or L_(LE)≤L_(TRGT)≤L_(TRAN)). Ifthe control circuit determines that it is not operating the loadregulation circuit in the burst mode (e.g., but rather in the normalmode), then the control circuit may determine and set the target loadcurrent I_(TRGT) as a function of the target intensity L_(TRGT) at 1414(e.g., as shown in FIG. 2). The control circuit may then set the burstduty cycle DC_(BURST) equal to a maximum duty cycle DC_(MAX) (e.g.,approximately 100%) at 1415 (e.g., as shown in FIG. 3), and the controlcircuit may exit the target intensity procedure 1400.

If the control circuit determines that it is operating the loadregulation circuit in the burst mode at 1412 (e.g., the target intensityL_(TRGT) is below the transition intensity L_(TRAN), orL_(TRGT)<L_(TRAN)), then the control circuit may determine the lengthsof the active state period T_(ACTIVE) and/or the inactive state periodT_(INACTIVE) for one or more burst mode periods T_(BURST) (e.g., usingopen loop control) at 1418. For example, the control circuit maydetermine target lengths of the active state period T_(ACTIVE) and theinactive state period T_(INACTIVE) that correspond to the targetintensity L_(TRGT). The control circuit may then determine the lengthsof the active state period T_(ACTIVE) and/or the inactive state periodT_(INACTIVE) for one or more burst mode periods. As described herein,the length of the inactive state period may be gradually adjusted (e.g.,gradually increased or decreased) in one or more burst mode periodsuntil a maximum amount of adjustment is reached. The length of theactive state period may then be adjusted in a subsequent burst modeperiod. The determination process may be repeated in the mannerdescribed herein until the target lengths of the active state periodT_(ACTIVE) and inactive state period T_(INACTIVE) are achieved.

The control circuit may perform the foregoing process by calculating therelevant values in real-time or retrieving the values from memory (e.g.,via a lookup table or the like). The control circuit may set the lengthsof the active state period T_(ACTIVE) and/or the inactive state periodT_(INACTIVE) for the one or more burst mode periods T_(BURST) at 1420,and the control circuit may exit the target intensity procedure 1400. Asdescribed herein, the control circuit may adjust the active state periodT_(ACTIVE) and/or the inactive state period T_(INACTIVE) as a functionof the target intensity L_(TRGT) using open loop control. Other ways toadjust the active state period T_(ACTIVE) and/or the inactive stateperiod T_(INACTIVE) may be employed, including, for example, usingclosed loop control (e.g., in response to the load current feedbacksignal V_(I-LOAD)).

One or more of the embodiments described herein (e.g., as performed by aload control device) may be used to decrease the intensity of a lightingload and/or increase the intensity of the lighting load. For example,one or more embodiments described herein may be used to adjust theintensity of the lighting load from on to off, off to on, from a higherintensity to a lower intensity, and/or from a lower intensity to ahigher intensity. For example, one or more of the embodiments describedherein (e.g., as performed by a load control device) may be used to fadethe intensity of a light source from on to off (e.g., the low-endintensity L_(LE) may be equal to 0%) and/or to fade the intensity of thelight source from off to on.

Although described with reference to an LED driver, one or moreembodiments described herein may be used with other load controldevices. For example, one or more of the embodiments described hereinmay be performed by a variety of load control devices that areconfigured to control of a variety of electrical load types, such as,for example, a LED driver for driving an LED light source (e.g., an LEDlight engine); a screw-in luminaire including a dimmer circuit and anincandescent or halogen lamp; a screw-in luminaire including a ballastand a compact fluorescent lamp; a screw-in luminaire including an LEDdriver and an LED light source; a dimming circuit for controlling theintensity of an incandescent lamp, a halogen lamp, an electroniclow-voltage lighting load, a magnetic low-voltage lighting load, oranother type of lighting load; an electronic switch, controllablecircuit breaker, or other switching device for turning electrical loadsor appliances on and off; a plug-in load control device, controllableelectrical receptacle, or controllable power strip for controlling oneor more plug-in electrical loads (e.g., coffee pots, space heaters,other home appliances, and the like); a motor control unit forcontrolling a motor load (e.g., a ceiling fan or an exhaust fan); adrive unit for controlling a motorized window treatment or a projectionscreen; motorized interior or exterior shutters; a thermostat for aheating and/or cooling system; a temperature control device forcontrolling a heating, ventilation, and air conditioning (HVAC) system;an air conditioner; a compressor; an electric baseboard heatercontroller; a controllable damper; a humidity control unit; adehumidifier; a water heater; a pool pump; a refrigerator; a freezer; atelevision or computer monitor; a power supply; an audio system oramplifier; a generator; an electric charger, such as an electric vehiclecharger; and an alternative energy controller (e.g., a solar, wind, orthermal energy controller). A single control circuit may be coupled toand/or adapted to control multiple types of electrical loads in a loadcontrol system.

What is claimed is:
 1. A circuit for controlling an intensity of a LEDlight source, the circuit comprising: an LED drive circuit configured tocontrol a magnitude of a load current conducted through the LED lightsource to control the intensity of the LED light source; and a controlcircuit configured to generate at least one drive signal for controllingthe LED drive circuit to control an average magnitude of the loadcurrent to adjust the intensity of the LED light source towards a targetintensity; wherein the control circuit is configured to: operate in afirst state and a second state on a periodic basis over a plurality ofburst periods, each of the plurality of burst periods including a firsttime period and a second time period; control the LED drive circuit inthe first state during the first time period in which the controlcircuit adjusts a value of an operational characteristic of the at leastone drive signal to regulate a peak magnitude of the load currenttowards a target current in response to a feedback signal; control theLED drive circuit in the second state during the second time period inwhich the control circuit maintains the operational characteristic ofthe at least one drive signal approximately constant; and adjust theaverage magnitude of the load current by adjusting a length of one ofthe first or second time period while holding a length of the other ofthe first or second time period constant in at least some of the burstperiods.
 2. The circuit of claim 1, further comprising: a current sensecircuit configured to generate the feedback signal, wherein the feedbacksignal comprises a load current feedback signal that indicates themagnitude of the load current.
 3. The circuit of claim 2, wherein, whenthe target intensity is greater than a transition intensity, the controlcircuit is configured to adjust the value of the operationalcharacteristic of the at least one drive signal in response to the loadcurrent feedback signal in order to regulate the average magnitude ofthe load current towards a target current.
 4. The circuit of claim 3,wherein, when the target intensity is greater than the transitionintensity, the control circuit is configured to hold the length of thefirst time period and the length of the second time period constant, andadjust the target current between a maximum rated current to a minimumrated current.
 5. The circuit of claim 4, wherein, when the targetintensity is less than the transition intensity, the control circuit isconfigured to adjust a duty cycle to adjust the average magnitude of theload current below the minimum rated current, the duty cycle definingwhen the LED drive circuit operates in the first state and the secondstate.
 6. The circuit of claim 4, wherein, when the target intensity isless than the transition intensity, the target current is approximatelyequal to the minimum rated current.
 7. The circuit of claim 4, wherein,when the target intensity is greater than the transition intensity, thecontrol circuit is configured to maintain the length of the second timeperiod at approximately zero.
 8. The circuit of claim 3, wherein thetransition intensity corresponds to a minimum rated current of the LEDdrive circuit.
 9. The circuit of claim 1, wherein the control circuit isconfigured to adjust the length of the second time period by an equalamount while holding the length of the first time period constant ineach of at least some of the burst periods.
 10. The circuit of claim 9,wherein the control circuit is configured to adjust the length of thesecond time period by an equal amount in each of at least some of theburst periods until a total amount of adjustment is approximately equalto a threshold value, the control circuit further configured to adjustthe length of the first time period by a first state adjustment amountin a subsequent burst period.
 11. The circuit of claim 10, wherein theamount of adjustment applied to the length of the second time period ineach of the at least some of the burst periods is smaller than the firststate adjustment amount.
 12. The circuit of claim 10, wherein thethreshold value is determined as a function of the length of the firsttime period in one of the plurality of burst periods.
 13. The circuit ofclaim 1, wherein the operational characteristic of the at least onedrive signal comprises a duty cycle of the at least one drive signal.14. The circuit of claim 1, wherein the operational characteristic ofthe at least one drive signal comprises an operating frequency of the atleast one drive signal.
 15. A method of controlling an intensity of aLED light source, the method comprising: generating at least one drivesignal for controlling an LED drive circuit to adjust an averagemagnitude of a load current conducted through the LED light source toadjust the intensity of the LED light source towards a target intensity;operating in a first state and a second state on a periodic basis over aplurality of burst periods, each of the plurality of burst periodsincluding a first time period and a second time period; controlling theLED drive circuit in the first state during the first time period inwhich a value of an operational characteristic of the at least one drivesignal is adjusted to regulate a peak magnitude of the load current inresponse to a feedback signal; controlling the LED drive circuit in thesecond state during the second time period in which the operationalcharacteristic of the at least one drive signal is maintainedapproximately constant; and adjusting the average magnitude of the loadcurrent by adjusting a length of one of the first or second time periodwhile holding a length of the other of the first or second time periodconstant in at least some of the burst periods.
 16. The method of claim15, wherein the feedback signal comprises a load current feedback signalthat indicates the magnitude of the load current.
 17. The method ofclaim 16, further comprising, when the target intensity is greater thana transition intensity, adjusting a value of the operationalcharacteristic of the at least one drive signal in response to the loadcurrent feedback signal in order to regulate the magnitude of the loadcurrent towards a target current that ranges from a maximum ratedcurrent to a minimum rated current.
 18. The method of claim 17, furthercomprising, when the target intensity is greater than the transitionintensity, holding the length of the first time period and the length ofthe second time period constant, and adjusting the target currentbetween the maximum rated current and the minimum rated current.
 19. Themethod of claim 18, further comprising, when the target intensity isless than the transition intensity, adjusting a duty cycle to adjust theaverage magnitude of the load current below the minimum rated current,the duty cycle defining when the LED drive circuit operates in the firststate and the second state.
 20. The method of claim 16, furthercomprising adjusting the length of the second time period by an equalamount while holding the length of the first time period constant ineach of at least some of the burst periods.